As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which exhibit high energy density and voltage, long lifespan and low self-discharge rate are commercially available and widely used.
As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
In particular, lithium manganese oxides, such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material, and thus are drawing much attention as a positive electrode active material that can replace LiCoO2.
However, since lithium manganese oxides such as LiMnO2 and LiMn2O4 cause rapid exhaustion of an electrolyte solution through reaction with an electrolyte solution, with increasing number of cycles, lifespan and cycle characteristics of secondary batteries may be deteriorated. In addition, the volumes of secondary batteries expand due to generation of a large amount of gas, elution of manganese, etc.
Meanwhile, as lithium-containing manganese oxides, there is Li2MnO3 in addition to LiMnO2 and LiMn2O4. Since structural stability of Li2MnO3 is excellent but it is electrochemically inactive, Li2MnO3 itself cannot be used as a positive electrode active material of secondary batteries. Therefore, some prior technologies suggest a technology of using a solid solution of Li2MnO3 and LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) as a positive electrode active material. In such a positive electrode active material solid solution, Li and O are separated from a crystal structure at a high voltage of 4.5 V and, thus, electrochemical activity is exhibited. However, there are unsolved problems such as exhaustion of an electrolyte solution and volume expansion of secondary batteries, due to easy reaction with an electrolyte solution at high voltage.
Accordingly, there is an urgent need for technology to inhibit reaction of an active material and an electrolyte solution and ultimately to improve lifespan and cycle characteristics of secondary batteries.